This invention relates to transmission lines wherein the wavelength of signals carried on the transmission lines is generally longer than the length of the transmission lines, such as in audio signal and high fidelity sound reproduction applications. The invention has particular application where the range of frequencies is greater than several octaves and therefore wherein spurious oscillations in a transmission line can have potentially significant impact on the fidelity of a complex signal carried by the transmission line.
In contrast to long transmission lines where the impedance of the cable is matched to the impedance of the termination loads, it is conventional in short transmission lines to reduce the impedance, and more particularly the resistance, of the cable to a minimum to reduce the electrical resistive loss in the cable. When used in high fidelity audio signal interconnection cabling, as between components in an audio component system, such an approach introduces secondary problems, such as audible enhancement of the "brightness" frequencies (1000 Hz to 2000 Hz range) and a loss in clarity in the high audio frequencies. The prior art in high fidelity audio cables is to provide means for varying the impedance of the cable over the frequency range to reduce the "brightness."
It has been found that one of the primary causes of spurious oscillation (ringing) is reflections in low loss interconnecting cables. The mechanism and extent to which the ringing interacts with the audio signal are not generally understood, and the significance of the interaction appear to have been underestimated in the audio high fidelity field. The ringing phenomenon causes audible enhancement of the "brightness" in the upper midrange, which exaggerates sibilance and transient attacks and causes distortion at high frequencies. Moreover it appears that the ringing causes standing waves in a transmission line which are not equal and opposite in amplitude and phase in the signal line and in the signal return line. Identification of this phenomenon led to the present invention.
The conventional solution for reducing radio frequency oscillations in transmission lines is to use frequency selective means in series with the signal lines. The equivalent of the frequency selective means is an r.f. choke. However, such an approach has several disadvantages. Impedance matching is required as well as sufficient frequency separation to allow attenuation of the spurious signal without attenuation of the desired signal. The present invention pursues an alternative approach.
The following patents and publications were uncovered in the course of research on the prior art of subject invention:
U.S. Pat. No. 4,538,023 issued Aug. 27, 1985 to Bruce A. Brisson describes an audio signal cable wherein impedance is varied over the frequency range to overcome phase distortion due to variation in group delay. The invention therein relates to a cable wherein the outer and inner conductors are constructed of helically wound wires of different lengths, the shorter wires being toward the center and wherein the impedance of the cable wires radially from a maximum at the center of the cable. This technology represents current commercially-available technology. This patent does not address the issue of reducing undesired ringing due to signal reflections. A cable derived from this construction is commercially available which is less subject to ringing wherein the cable is constructed with tuning stubs at each end.
U.S. Pat. No. 687,141 issued Nov. 29, 1901 to Everest describes a current transformer with a secondary circuit being wound around a magnetizable core and a primary consisting of a single conductor. Although superficially similarly constructed to the present invention, current transformers are designed to produce an output from the secondary which is used for measurement or external control. As will be apparent hereinafter, the present invention uses effects produced by a secondary for suppression of spurious currents in the primary.